The stripping of organic dye molecules from textiles (e.g., woven, non-woven, and knitted cloth, and yarn) is performed when, for example, a first dyeing is unlevel and the dye house operator wishes to remove it preparatory to a second dyeing. The dye does not merely cling to the surfaces of the textile fibers but is held in the amorphous regions of fiber matrices by ionic and covalent bonds as well as by van der Waal forces and hydrogen bonding. Sodium dithionite and thiourea dioxide (also known as form amidine sulfonic acid or FAS) are the leading dye stripping agents in the textile industry. Sodium dithionite (also known as sodium hydrosulfite or "hydro") is the most popular one and is most often used as a powder containing 70% by weight of the active material along with sodium carbonate as a fire retardant, a chelating agent such as EDTA or a sequestering agent such as STPP. The textile industry still suffers from many spontaneous fires and the concomitant emission of sulfur dioxide when the dithionite powder comes into contact with moisture. Although use of an aqueous solution of the dithionite is growing because of the health, flammability, storage, and handling problems involved with the use of the powder, the solution must be stored under an inert atmosphere in dedicated insulated tanks that must be refrigerated for long term storage. Despite those problems and the recognition that the dithionite powder is inefficient and unreliable, it has retained its standing as the preferred stripping and reducing agent.
In conventional practice, the textile is placed in an aqueous bath, the pH of the bath is adjusted according to the requirements of the particular kind of textile to be stripped (i.e., below 7.5 for acetate and from a minimum of 10.2 to about 12.3 for cotton), and the bath is then heated to about 90-160.degree. F. before the dithionite powder is added. The stripping bath is then heated to 212.degree. F. ("the boil") or as high as about 230.degree. F. when a pressurized vessel is used. The stripping is continued at temperature for about 30 minutes or more, the bath is dropped, and the stripped textile is rinsed twice. Oftentimes, the procedure must be repeated to achieve the necessary dye stripping and color removal.
It is clear, therefore, that the industry needs a more efficient and more reliable stripping system that gives satisfactory color removal even when the temperature is below the boil and the stripping time is much less than the conventional 30 minutes.